Traditionally, backing films such as biaxially stretched polypropylene films (hereinafter abbreviated as OPP), biaxially stretched polyamide films (hereinafter abbreviated as OPA), and biaxially stretched polyethylene terephthalate films (hereinafter abbreviated as PET) which are superior in transparency, mechanical strength, price, etc. are laminated with polyolefin films for imparting heat sealing property, and used as packaging materials for various uses such as for foods, medicines, and so on. However, these films have poor in gas barrier properties, which in turn results in such poor storage of contents as exemplified by the formation of mold on the contents, volatilization of the contents, oxidation of fats and oils, and degradation of flavor. Accordingly, an improvement in gas barrier properties is desired.
Improvement of the gas barrier properties can be achieved by coating a latex of a vinylidene chloride--vinyl chloride copolymer or a solution of a vinylidene chloride--acrylonitrile copolymer on a film (commercially available from various companies as PVDC coated films), and by laminating the film with a film made of ethylene--vinyl alcohol copolymer (hereinafter abbreviated as EVOH) or a film made of poly(vinyl alcohol) (hereinafter abbreviated as PVA). However, PVDC coated films are yellowy as they originally are, and can be further yellowed by ultraviolet rays.
On the other hand, laminates of films of EVOH and PVA are colorless and transparent, and do not yellow upon exposure to ultraviolet radiation. However, they have defects in that they cost more, they have poor flex resistance due to the hardness of the film, and delamination tends to occur upon boil-treatment at high temperature.
Japanese Patent Unexamined Publication No. 18775/1976 discloses a method for producing a laminated film, which comprises laminating a thin molten film consisting of at least two layers with one layer being EVOH, with a backing film by contact bonding. However, while this invention teaches coating of an isocyanate adhesive (adhesion promoter) for the adhesion between the backing film and a co-extrusion coating layer in Examples thereof, it does not teach co-extrusion coating without an adhesion promoter. For using an adhesion promoter, a coater and a drying machine will be necessary. Then, the equipment becomes complicated, the cost becomes higher, and high-speed operation becomes unattainable. In addition, this invention does not teach that a multi-layer construction film having a flex resistance as measured by Gelbo flex tester (an equipment according to Military standard, MIL-B-131G, METHOD 2017) similar to that of the backing film can be obtained by laminating an adhesive resin having a density of not higher than 0.90 g/cm.sup.3 which is made of ethylene polymer, and an EVOH having a ratio of melt index in a specific range at a specific layer thickness, and by reducing the thickness of not only the EVOH layer but also the adhesive resin layer. On the contrary, this invention teaches that the adhesive resin layer should be made thicker for the adhesion with the film, since heat capacity of the molten resin needs to be increased.
Japanese Unexamined Patent Publication No. 119690/1977 discloses a laminated material comprising a thermoplastic resin layer having a tensile modulus of not more than 100 kg/mm.sup.2 between a paper layer and an EVOH layer. While this invention teaches adhesion of the paper layer and the EVOH layer, it does not teach that of plastic films. In addition, this invention has no disclosure as regards laminating without an adhesion promoter, or flex resistance.
Japanese Unexamined Patent Publication No. 182435/1982 discloses a method for producing a laminate composed of polyolefin layer/modified polyolefin layer made of carboxyl group-containing polyolefin or those obtained by adding a metal compound to the carboxyl group-containing polyolefin/EVOH layer/adhesive layer/polyolefin layer, which comprises melt extrusion or co-extrusion of one to three layers of EVOH, modified polyolefin and EVOH, EVOH and adhesive, and modified polyolefin and EVOH and adhesive, for sandwich laminating. However, this invention does not describe co-extrusion coating or flex resistance.
As mentioned above, a low-cost multi-layer construction film having gas barrier properties, flex resistance, and resistance to boiling, which is scarcely yellowed by ultraviolet rays is desired in this industrial field.